The present invention relates to a method for economically converting salty or brackish water to fresh water by utilizing the existing water pressures that are available in operations such as the injection of water for oil recovery.
There is an increasing need for fresh water in various parts of the world, and a number of these areas are near operations where large amounts of water are pumped at pressures that can be utilized to purify water by reverse osmosis. Although reverse osmosis is widely used to remove dissolved solids from various impure waters, a few inherit problems limit its overall effectiveness. Chief among these are:                1. The energy cost for pumping the raw feedwater up to pressures sufficient for reverse osmosis.        2. The disposal of the reject water, whose saltiness is much increased over that of the original feedwater.        3. Membrane fouling, both from entrained solids and from a buildup of the rejected ions on the membrane surface.        
Current reverse osmosis practices address these difficulties in various ways, but it is believed that none have solved all of the problems as effectively and economically as is proposed by the present invention.
Disposal of the reject water for reverse osmosis remains a problem, especially in areas where laws restrict its discharge into streams, underground, or on the surface. To overcome the problem of the discharge of large volumes of salty water, U.S. Pat, No. 6,241,892, T. M. Whitworth, provides a process wherein the rejected material consists primarily of the solid salts, but even these small volumes must be disposed of or utilized in some way.
Membrane fouling is always a limitation in reverse osmosis and is often handled by simply halting the operation and using a procedure such as flushing to clean the surface of the membranes, but this interruption reduces the overall conversion efficiency of the feedwater to fresh water.
Therefore, there have been many attempts to improve on the membrane flushing/cleaning systems, or to pre-treat the feedwater so flushing is not needed as often.
For example, U.S. Pat. No. 6,334,955, T. Kawashima and T. Kawada, requires a timer that periodically opens and closes a special flushing valve to clean the reverse osmosis membranes during fresh water generation work, when interrupted, or when restarting after interruption, but the device adds to the complexity and cost of the system.
A number of physical and chemical methods have been studied to improve the quality of various feedwaters prior to contact with the reverse osmosis membranes. By way of example:                U.S. Pat. No. 6,395,181, S. B. Mullerheim, uses physical separation methods to remove solids and floatable materials;        U.S. Pat. No. 6,365,051, M. S. Bader, adds an organic solvent to precipitate the dissolved salts;        U.S. Pat. No. 6,183,646, E. E. Williams et al., treats the feedwater with agents designed to prevent biofouling;        U.S. Pat. No. 5,925,255, D. Mukhopadhyay, uses special treatments at high pH to remove hardness, etc. from feedwater prior to reverse osmosis;        U.S. Pat. No. 5,250,185, F. T. Tao, et al., softens the feedwater and raises the pH prior to reverse osmosis to reject more of the boron.        
Thus, although there have been many efforts to try to solve certain facets of the aforementioned reverse osmosis problems, all known methods would add to the cost of the fresh water produced, and none of the improvements can solve all of the problems at the same time.
It is therefore an object of the present invention to utilize the existing pressures and high-flow velocities, which are available in situations such as waterflood injection water systems, to produce fresh water by reverse osmosis at a very low cost, with a minimum of membrane fouling, and with no wastewater disposal problems.